Gap distributed Bragg reflectors

ABSTRACT

A device includes one or more reflector components. Each reflector component comprises layer pairs of epitaxially grown reflective layers and layers of a non-epitaxial material, such as air. Vias extend through at least some of the layers of the reflector components. The device may include a light emitting layer.

RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 13/217,859 filed Aug.25, 2011, the contents of which is incorporated herein by reference inits entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under U.S. ArmyCooperative Agreement No. W911NF-10-02-0102 awarded by the DefenseAdvanced Research Projects Agency (DARPA). The Government has certainrights in this invention.

SUMMARY

Approaches involving gap distributed Bragg reflectors (DBRs) arediscussed in this disclosure. Some embodiments illustrate a method ofmaking a semiconductor light emitting device. A light emitting layer,one or more reflective layers, and one or more interstitial layers areepitaxially grown. Each interstitial layer is disposed between tworeflective layers. Vias are formed that intersect at least some of thelayers. The interstitial layers are exposed to an etchant through thevias and are etched.

Some embodiments are directed to a semiconductor light emitting device.The device includes a light emitting layer and one or more reflectorcomponents. Each reflector component comprises alternating epitaxiallygrown reflective layers and layers of a non-epitaxially grown material.Vias extend through at least some of the layers of the reflectorcomponents.

The above summary is not intended to describe each embodiment or everyimplementation. A more complete understanding will become apparent andappreciated by referring to the following detailed description andclaims in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically illustrates etch times of epitaxial Al_(x)Ga_((1-x))Nlayers in phosphoric acid;

FIG. 2 is a graph of the etch rate of Al_(x)Ga_((1-x))N layers inphosphoric acid;

FIG. 3 is a flow diagram illustrating a method of forming a devicecomprising a gap distributed Bragg reflector (DBR) in accordance withembodiments described herein;

FIG. 4 is a diagram that illustrates a process for forming a gap DBRbefore and after the removal of the interstitial layers in accordancewith embodiments described herein;

FIG. 5 is a flow diagram that shows a process for forming a mesastructure that allows access to the sidewalls of the interstitial layersby an etchant in accordance with various embodiments;

FIG. 6 is an example of a mesa structure that exposes at least onesidewall of the interstitial layers in accordance with embodimentsdescribed herein;

FIG. 7 is another example of a mesa structure that exposes at least onesidewall of the interstitial layers in accordance with embodimentsdescribed herein;

FIG. 8 is a flow diagram that illustrates a process for forming a viasthat expose the interstitial layers to an etchant in accordance withvarious embodiments;

FIG. 9 illustrates a device with one or more reflective layers and oneor more interstitial layers in accordance with embodiments describedherein;

FIG. 10 shows a device with one or more reflective layers, one or moreinterstitial layers, and vias that expose the interstitial layers to anetchant in accordance with embodiments described herein;

FIG. 11 is a device with one or more reflective layers and air gapssandwiched between the reflective layers in accordance with embodimentsdescribed herein;

FIG. 12 illustrates a device with vias that are oriented to surround acentral region in accordance with embodiments described herein;

FIG. 13 shows an example in which the vias have a linear orientation andare oriented in one or more rows on the device in accordance withvarious embodiments;

FIG. 14 illustrates a device with a light emitting layer, one or morereflective layers, and one or more interstitial layers in accordancewith various embodiments;

FIG. 15 is a device with a light emitting layer, one or more reflectivelayers, and one or more air gaps oriented between the reflective layersin accordance with various embodiments;

FIG. 16 shows a flow diagram of a method of making a device with a lightemitting layer in accordance with various implementations.

FIG. 17 is a schematic diagram of a device having a light emitting layerand two epitaxially-grown reflective regions in accordance withembodiments described herein;

FIG. 18 is a flow diagram of a method of making a device with a lightemitting layer in accordance with embodiments described herein;

FIG. 19 is a schematic diagram of a device having an active region, anepitaxially grown reflective region, and a dielectric reflective regionin accordance with various embodiments;

FIG. 20 illustrates a device in which one set of vias allows access byan etchant to the interstitial layers of two reflective regions inaccordance with embodiments described herein;

FIG. 21 shows an example of a device in which two sets of vias are usedto expose the interstitial layers from the two reflective regions inaccordance with embodiments described herein;

FIG. 22 shows another example of a device in which two sets of vias areused to expose the interstitial layers from the two reflective regionsin accordance with various embodiments;

FIG. 23 is a flow diagram of a method for controlling an etch process inaccordance with various embodiments;

FIG. 24 is a diagram that illustrates a camera-based control system foran etch process in accordance with embodiments discussed herein; and

FIG. 25 is a diagram that illustrates a reflectivity-based controlsystem for an etch process in accordance with embodiments describedherein;

FIG. 26 illustrates a sensor system including a DBR sensor in accordancewith various embodiments; and

FIG. 27 shows a monolithic layered structure that forms a DBR sensorsystem in accordance with embodiments described herein.

DESCRIPTION OF VARIOUS EMBODIMENTS

Various embodiments involve highly reflective distributed Braggreflectors (DBRs) and processes for fabricating such DBRs. The DBRs maybe useful in vertical cavity surface emitting laser (VCSEL) structures,for example. High reflectivity in the mirror components can provide alower lasing threshold in semiconductor laser devices. Enhancedreflectivity may be achieved in the DBRs described herein due to therelatively high difference in refractive index provided by thealternating layers. In some embodiments, the DBRs are formed using analternating stack of reflective and interstitial layers that can beselectively etched.

Embodiments described in this disclosure involve DBRs with interstitialgaps between reflective layers, methods of creating the DBRs, andsemiconductor light-emitting devices that include the DBRs. For example,gap DBRs may be formed by growing etchable interstitial alternating withlayers that are not etched. At least one sidewall of the interstitiallayers is exposed to an etchant. The etchant at least partially removesthe interstitial layers between the non-etched reflective layers forminginterstitial gaps between the reflective layers. In some embodiments,removal of the interstitial layers involves creating via holes to accessthe interstitial layers so that they can be exposed to the etchant. Airmay occupy the interstitial gaps after the etching.

In DBRs that do not include air gaps, a relatively large number ofmirror pairs may be needed because the refractive index differencebetween the alternating layers is relatively low. Having a large numberof mirror pairs may increase the formation of defects and/or may causethe structure to crack due to a high lattice mismatch between the layersof the DBR. The relatively high refractive index difference in a devicewith nitride-containing layers (e.g., group III-nitrides—(Al—, In—, Ga—,B—)N, or various combinations group III materials and N, such as, AlGaN,InGaN, and InAlGaN, alternating with air gaps can provide a higheroverall reflectivity and/or can allow for a reduction in the number ofmirror pairs needed to achieve a similar or greater amount ofreflectivity than a device without air gaps.

The devices formed according to the approaches described herein can havea reflectivity greater than 99% with fewer than 20 reflector pairs oreven fewer than 5 reflector pairs. For example, the reflectivity, R, atthe interface between an AlN and a GaN layer at a wavelength of 350 nmis less than about 1%, whereas at a GaN/air interface, R is as high asabout 21.3% (the index of refraction for GaN is n_GaN=2.713, the indexof refraction for AlN is n_AlN=2.225, and the index of refraction forair is about 1). The difference in refractive index may be greater than0.6 for layer pairs that include an epitaxially grown reflective layerand a non-epitaxial material disposed in the interstitial gaps betweenthe reflective layers. In some cases, the second material may be air. Insome cases, there may be a vacuum in the interstitial gaps.

In some implementations, a material other than air may be present in thegaps between the reflective layers. For example, the DBR may beconfigured to function as a sensor wherein the optical characteristicsof the DBR properties change based on a presence and/or one or morecharacteristics (e.g., temperature, pressure, or concentration) of ananalyte. DBR sensors and systems are described more fully below.

In examples described below, alternating layers of GaN and etchableAlGaN are described as the reflective and etchable interstitial layers,respectively. It will be appreciated that other materials may be used tocreate the DBRs discussed herein so long as the material of theinterstitial layer can be selectively etched while the material of thereflective layer remains intact. Generally, the interstitial layers maybe referred to as etchable layers and the reflective layers may bereferred to as non-etchable layers. This terminology is intended toconvey that the etchable layers are relatively more easily etchable thanthe non-etchable layers using the etchants and/or etching processesdescribed herein.

Selectively etching AlN or AlGaN in the DBR structures is possiblebecause of the differential etch rates of GaN (or AlGaN having lowaluminum content) as compared to AlN (or AlGaN layers having higheraluminum content). The etch rate of AlGaN materials varies as a functionof aluminum content. FIGS. 1 and 2 are graphs illustrating etch timesand etch rate, respectively, of epitaxially grown AlGaN layers as afunction of mole fraction of aluminum. For each sample, the thickness ofthe AlGaN layer being etched was monitored by periodically taking thesample out of the chemical bath etchant and performing x-ray scans todetermine the x-ray signal intensity of the AlGaN layer. FIG. 1 showsetch times of epitaxial Al_(x)Ga_((1-x))N layers having a thickness ofabout 1.9 μm etched in phosphoric acid (H₃PO₄) at a concentration 92%and temperature of 180° C. The layers of AlGaN having aluminum molefraction of 74% or 67% were substantially removed in less than 5 min.,whereas the AlGaN layer having 47% aluminum mole fraction took over 30min. to remove.

FIG. 2 shows a graph of the etch rate of Al_(x)Ga_((1-x))N in H₃PO₄ at aconcentration of 92% and temperature of 180° C. as a function ofaluminum mole fraction. The graph shows that the etch rate increaseswith increasing aluminum mole fraction.

FIG. 3 is a flow diagram illustrating a method of forming a devicecomprising a gap DBR in accordance with embodiments described herein. Astack of alternating etchable interstitial layers and non-etchablereflective layers is epitaxially grown 310. The stack is exposed 330 toan etchant which etches the interstitial layers. The interstitial layersare etched 340 for a predetermined period of time. During this timeperiod, the interstitial layers may be partially or fully removed.Removal of the interstitial layers creates interstitial gaps between thereflective layers, which may be occupied by a second material, such asair. The reflective and/or interstitial layers may comprise groupIII-nitride materials such as (Al—, Ga—, In—, B—)N or any combination ofgroup III elements and nitrogen, e.g., AlGaN, InGaN, InAlGaN, etc. Insome cases, the one or more reflective layers comprise gallium nitride(GaN) and the one or more interstitial layers comprise aluminum galliumnitride (AlGaN) having an aluminum content sufficient to allow etching.The GaN/air gap pairs formed by the etching can provide a higherrefractive index difference as compared to the refractive indexdifference achieved with GaN/AlGaN pairs. As previously mentioned, thehigher refractive index contrast between the reflective layers and theair gaps allows a device to be grown with fewer reflective layers for asimilar and/or greater amount of reflectivity.

FIG. 4 is a diagram that illustrates a gap DBR before and after theremoval of the interstitial layers. Before removal of the interstitiallayers, the device 400 a includes one or more interstitiallayer/reflective layer pairs 440/430. The device 400 may include one ormore additional layers 420. The additional layers 420 can provide a basefor the epitaxial growth of the interstitial layer/reflective layerpairs 440/430 and/or the additional layers 420 can be grown on theinterstitial layer/reflective layer pairs 440/430. Four interstitiallayer/reflective layer 440/430 pairs are shown in the device 400 a ofFIG. 4. The interstitial layers 440 may be selectively etched away by awet etchant such as hot phosphoric acid, as described previously. Afterremoval of the interstitial layers 440 by the etching process,interstitial gaps form between the reflective layers 430. Theinterstitial gaps (which may be occupied by a material such as air) aresandwiched between reflective layers 430 make up a reflective region 445of the device 400 b. The reflectivity of region 445 b of device 400 b isgreater than the reflectivity of region 445 a of device 400 a. Thethickness of the reflective and air layers is related to the targetwavelength of interest. In general, the target wavelength can be anywavelength, however, a target wavelength range of 350 to 600 nm is ofinterest for many implementations. For example, the layer thicknessd=(m*λ)/(4*n), where m is equal to 1,3,5,7, λ is the target wavelength,and n is the refractive index. For blue light (λ=460 nm) and GaN/AlGaNreflective/interstitial layers, the thickness of a GaN reflective layerd_GaN=460/(4*2.476)=46.4 nm and the thickness of the AlGaN interstitiallayer d_AlGaN=d_air=460/4=115 nm.

The ability to etch the interstitial layers may rely on accessing atleast one sidewall of interstitial layer so that each interstitial layercan be exposed to the etchant. One way to access sidewalls of theinterstitial layers is to epitaxially grow interstitial layer/reflectivelayer pairs 510 followed by formation of a mesa structure 520, asillustrated by the flow diagram of FIG. 5. Formation of the mesastructure may involve etching a mesa structure, e.g., mesa stripe, intothe epitaxially grown layers. Etching the mesa stripe is accomplished bynon-selective etching of both the reflective and interstitial layers.The mesa structure allows access to sidewalls of the interstitiallayers. The sidewalls of the interstitial layers (made accessible by themesa structure) are exposed 530 to a wet etchant during an etchingprocess that selectively etches the interstitial layer and leaves thereflective layers intact. Formation of a mesa structure 610 may allowaccess to two sidewalls 620, 630 of the interstitial layers as shown inFIG. 6. Alternatively, only one sidewall 720 of the interstitial layersmay be accessed by formation of a step structure 710 as illustrated inFIG. 7.

In some processes, access to the interstitial layers may be achievedthrough one or more via holes as illustrated in the flow diagram of FIG.8. Pairs of reflective layers and interstitial layers are epitaxiallygrown 810. One or more vias are formed 820 that penetrate the reflectivelayers and the interstitial layers. The one or more vias can be createdusing a non-selective dry etching process. For example, the vias may becreated by using electron cyclotron resonance (ECR), inductively coupledplasma (ICP), reactive ion etching (RIE), chemically-assisted reactiveion etching (CAME), and/or processes. The creation of the one or morevias allows access to the interstitial layers which are exposed 830 to awet etchant, e.g. phosphoric acid. The interstitial layers are partiallyor fully etched 840 by the etchant. In some cases, interstitial layersmay be etched for a period of time that is selected to achieve apredetermined amount of etching. The characteristics of the layer stackmay be used to control the etching process. For example, in someimplementations, the color and/or reflectivity of the layer stack ismonitored during the wet etching process and etching is controlled,e.g., the etching rate may be decreased or stopped once a certain colorand/or reflectivity is reached.

FIGS. 9-11 illustrate a process for creating vias to expose one or moreinterstitial layers to an etchant in accordance with embodimentsdescribed herein. FIG. 9 illustrates a layer stack having alternatingreflective layers and interstitial layers. FIG. 10 shows the creation ofvias 1010 in the layer stack of FIG. 9. The vias can be etched using anon-selective etching process, as described previously, e.g., a wet ordry etching process, which etches both the reflective and non-reflectivelayers. As can be observed from FIG. 10, the vias allow access to thesidewalls 1011 of the layer stack. FIG. 10 shows the two vias 1010, butit will be appreciated that more or fewer vias may be created. In somecases, the vias may only penetrate through some of thereflective/interstitial layer pairs in the layer stack. Exposing asidewall 1011 of the interstitial layers may alternatively oradditionally include creating vias in conjunction with the creation of amesa structure such that a first subset of the interstitial layers havea sidewalls that are accessed by the creation of the mesa structure anda second subset of the interstitial layers are accessed through thecreation of vias. FIG. 11 illustrates a cross-section of a layer stackafter the one or more interstitial layers have been etched by exposureof the interstitial layers to an etchant that flows into the vias. Theinterstitial layers are replaced by interstitial gaps, e.g., air gapsthat have a higher refractive index difference with the reflectivelayers, thus increasing the reflectivity of the layer stack.

The one or more vias may be formed in any arrangement and may be createdthrough any surface of the device. FIG. 12 shows a top view 1201 of adevice. In this example, the vias 1210 are oriented to surround acentral region 1205 of the device. The vias may be created on the topand/or the bottom surfaces of the device, for example. The orientationshown in FIG. 12 provides a high reflectivity area in the central region1205 after the interstitial layers are etched. In some cases, the one ormore vias may be oriented in a linear pattern along one or more sides ofthe device. FIG. 13 illustrates an example in which the vias have alinear orientation and are oriented in one or more rows along a surface1301 of the device. Note that the device may have more or fewer viasthan as depicted in FIGS. 12 and 13

Various devices described herein include a light emitting-layer anaddition to one or more reflective regions. FIGS. 14 and 15 illustrate adevice having a light emitting layer between two reflective regions inaccordance with embodiments described herein. In FIGS. 14 and 15, thereflective layers comprise GaN and the interstitial layers compriseAlGaN. Additionally or alternatively, the reflective and/or theinterstitial layers may comprise other materials. FIG. 14 illustratestwo regions before etching having alternating GaN reflective layers andAlGaN interstitial layers. The light emitting layer comprises threequantum wells. One of the reflective regions is below the light emittinglayer and one is above the light emitting layer.

The interstitial layers can be exposed to a wet etchant by exposingsidewalls by the formation of a mesa structure and/or the formation ofvias that intersect the interstitial layers, for example. FIG. 15illustrates the device of FIG. 14 after the interstitial layers areetched. The interstitial layers may be exposed to the etchant for aperiod of time based on etch rate. As can be observed in FIG. 15,interstitial gaps are left in the place of the AlGaN. The interstitialgaps can be occupied by air to form air gap DBRs 1410 with thereflective GaN layers.

FIG. 16 is a flow diagram of a method of making a device with a lightemitting layer in accordance with various embodiments. A device with alight emitting layer and a first reflective region is grown 1610 in afirst epitaxial growth session. The first reflective region comprisesone or more reflective layers and one or more interstitial layers.Optionally, an isolator layer is deposited 1620 that provides currentconfinement within the device. The isolator layer may comprise SiO₂, forexample, and is deposited using evaporation and/or sputteringtechniques. In some cases, a thin interstitial layer is grown and/ordeposited in place of the isolator. The interstitial layer can bepartially etched and/or oxidized to enhance current confinement. Asecond reflective region having one or more reflective regions and oneor more interstitial regions is grown 1630 in a second epitaxial growthsession. The interstitial layers of the first and the second reflectiveregions are etched 1640.

The process of FIG. 16 can be used to create a device depicted in FIG.17, for example. The heterostructure illustrated in FIG. 17 may be grownby metal organic vapor epitaxy to include a multiple quantum well (MQW)light-emitting layer 1710, two epitaxially-grown reflective regions1720, 1730 and an isolator 1740. As described in the discussion of FIG.16, the first reflective region 1720 and the light emitting layer 1710are grown in a first epitaxial grown session. The isolator 1740 isdeposited and a second epitaxial growth session creates the secondreflective region 1730. FIG. 17 shows the reflective regions 1720, 1730with the interstitial layers etched. In some cases, one etching processetches the interstitial layers for the first reflective region 1720 anda second etching process etches the layers for the second reflectiveregion 1720. For example, the first etching process may etch theinterstitial layers for the first reflective region 1720 before thegrowth of the light emitting layer and the second etching process etchesthe interstitial layers in the second reflective region 1730.

FIG. 18 is a flow diagram of a method of making a device with a lightemitting layer in accordance with various implementations. A device witha light emitting layer and a first reflective region is grown 1810. Thefirst reflective region comprises one or more reflective layers and oneor more interstitial layers. An isolator layer and a second reflectiveregion are deposited 1820, 1840. A contact layer, e.g., comprising ITO,for example, may also be deposited. The isolator layer may comprise SiO₂and the second reflective region may comprise alternating layers of SiO₂and TiO₂, for example. The interstitial layers of the first reflectiveregion are etched 1830. The second reflective region is deposited usingevaporation or sputtering, for example. The interstitial layers of thefirst reflective region may be etched before or after the deposition ofthe second reflective region.

FIG. 19 illustrates a device that may be fabricated using the method ofFIG. 18. The device of FIG. 19 comprises a light-emitting layer 1810, afirst reflective region 1830 that is epitaxially grown, and a secondreflective region 1820 that is deposited. As described above, the secondreflective region 1820 may be a dielectric DBR comprising alternatingdeposited layers of SiO₂ and TiO₂, and/or other materials. Theinterstitial layers in the first reflective region 1830 may be etchedbefore or after the growth of the light emitting layer and/or thedeposition of the second reflective layer.

FIGS. 20-22 illustrate examples of via formation to access interstitiallayers in a layer stack. FIG. 20 shows a device having a firstreflective region 2010, a light emitting layer 2030, and a secondreflective region 2020. According to FIG. 20, the first reflectiveregion 2010 comprises one or more reflective layers 2012 alternatingwith one or more interstitial layers 2014 and the second reflectiveregion 2020 comprises one or more reflective layers 2022 alternatingwith one or more interstitial layers 2024. A set of vias 2040 are etchedto allow access to the interstitial layers by an etchant. In the case ofFIG. 20, the vias are created through the top of the device and allowaccess to the set of interstitial layers 2014 in the first reflectiveregion 2010 and the set of interstitial layers 2024 in the secondreflective region 2020. In this example, the vias go through the lightemitting layer 2030. Additionally or alternatively, vias may be etchedon the bottom side of the device in order to expose the interstitiallayers.

FIG. 21 illustrates reflective regions 2110, 2120 and light emittinglayer 2130 which are epitaxially grown. A mesa structure is formedallowing vias to be etched to allow access to the interstitial layers ofthe first reflective region 2140 that do not penetrate the lightemitting layer 2130. In some cases, the mesa structure is formed byusing a non-selective dry etching process. The mesa structure may alsobe formed during the growth of the device by masking to preventepitaxial growth on surfaces at either side of the mesa structure. Thesecond reflective region 2120 may have a different set of vias 2145 thatallow access to the interstitial layers 2124 as shown in FIG. 21.

FIG. 22 shows another example of a device that includes first and secondreflective regions 2210, 2220 and a light emitting layer 2230. Again,the first and second reflective regions 2210, 2220 each comprise one ormore reflective layers 2212, 2222 alternating with one or moreinterstitial layers 2214, 2224. In this example, the device is grown andtwo sets of vias 2240, 2245 are formed that allow access to theinterstitial layers 2214, 2224 in the first and the second reflectiveregions 2210, 2220. A first set of vias 2240 are formed below the lightemitting layer 2230 penetrating the interstitial layers 2214 of thefirst reflective region 2210. In some cases, a substrate is removedbefore creation of the vias 2240. A second set of vias 2245 are formedabove the light emitting layer 2230 that expose the interstitial layers2224 of the first reflective region 2220 to an etchant.

In some implementations, processes and/or systems used to formreflective regions by etching interstitial layers may involve monitoringand/or controlling the etching process based on a predetermined limit.The system may be fully or partially automatic. In various scenarios,the limit may involve a time period or a detected device characteristic.FIG. 23 is a flow diagram illustrating etch process control. Thesidewalls of the interstitial layers of a layer stack are accessed 2330,e.g. by formation of a mesa structure and/or vias. The interstitiallayers are exposed 2340 to the etchant. Periodically, the process checksto see of the etch limit has been reached. The etch limit can be aperiod time, or a device characteristic such as reflectivity of thelayer stack or color of the layer stack for example. If the etch limitis reached, the etch process is controlled 2360, e.g., slowed orterminated.

FIGS. 24 and 25 show systems for etching layer stacks in accordance withvarious embodiments. Both systems include an etchant tank 2410configured to contain an etchant 2420. A layer stack 2430 can besubmerged in the tank 2410 so that interstitial layers are exposed tothe etchant 2420. The system of FIG. 24 includes a control system 2455that includes a camera 2440. The camera 2440 may include image processorcircuitry capable of discerning color changes in the surface of thelayer stack 2430 and generating a signal based on color of the layerstack surface. The signal is used by the process control unit 2450 tomonitor and/or control the etch process. For example, in some cases,when the signal from the camera 2440 indicates a change in color thatexceeds a predetermined etch limit, the process control unit 2450generates an alert signal and/or terminates the etch process. Thecontrol system 2555 of FIG. 25 includes a light source 2550 and detector2560. Light from the light source 2550 is reflected by the layer stack2430 during the etch process. The reflectivity of the layer stack 2430increases as the etching proceeds. Reflected light is detected by adetector 2560 which generates a signal indicative of the reflectedlight. The process control unit 2570 uses the signal to monitor and/orcontrol the etch process as described above.

In some cases, the DBR structures described herein may be used assensors and/or may be coupled to other components to form sensingsystems. The DBR may be configured so that the optical properties of theDBR, e.g. transmissive and/or reflective properties of the DBR, varybased on the presence and/or characteristics of an analyte occupying inthe interstitial gaps between the reflective layers. In some cases, theDBR may be coupled with a laser and properties of the laser, e.g.,lasing wavelength, are changed based on the presence and/orcharacteristics of the analyte.

FIG. 26 illustrates a sensing system 2600 that relies on a DBR sensor2610. The DBR sensor may include one or more material layers in additionto reflective layers. For simplicity, only the reflective layers and theinterstitial gaps between the reflective layers of the DBR are shown inFIG. 26. In some cases, the one or more additional material layers maybe deposited above and/or below the DBR portion of the sensor. In somecases the DBR and additional layers may form a light emitting devicesuch as a laser.

As illustrated by the system 2600, a DBR sensor 2610 includes of aseries of reflective layers and interstitial gaps between the reflectivelayers. One or more openings 2611 allow entry of an analyte into theinterstitial gaps. The system 2600 includes a light source 2620configured to direct light 2621 toward the DBR sensor 2610. The lightsource may provide broadband or narrow band light that includes lighthaving the target wavelength of the DBR sensor 2620. Light reflected2622 by the DBR sensor 2610 is detected by a photodetector 2630 whichgenerates a signal 2631. When an analyte enters through the openings2611 and into the interstitial gaps between the reflective layers, therefractive index difference between the DBR layers changes. The changein the refractive index difference in turn alters the optical propertiesof the DBR. For example, the reflectivity of the DBR sensor 2610 maydecrease or increase and/or the transmission through the DBR sensor 2610may increase or decrease. The signal 2631 output from the photodetector2630 is indicative of the changes in the DBR sensor optical properties.In some implementations, the analyte may be identified based on changesin the signal 2631. For example, if the refractive index of the analyteis known, then the analyte may be identified based on the change in thereflectivity and/or transmissivity of the DBR sensor 2610. In somecases, the sensor system may use a reference cell that is identical inall respects to the DBR sensor except that there are no openings to theinterstitial gaps that allow entry of the analyte. The reference cellmay be placed in the test environment with the sensor. The system maycompare the signal produced by the reference cell to the signal producedby the sensor.

FIG. 27 illustrates yet another example of a DBR sensor system 2700. Inthis example, the sensor system 2700 is a monolithic layered structure.A first set of layers forms the DBR sensor portion 2710 includingreflective layers and interstitial gaps. Openings 2711 allow analyte toenter at least some of the interstitial gaps. The sensor system 2700includes light emitting layers 2720 configured to emit light. In somescenarios the light emitted by the light emitting layers 2720 has awavelength range of about 300 to about 600 nm, although other wavelengthranges may be used in other applications. The sensor system 2700includes photodetector layers 2730, which, according to the orientationshown in FIG. 27, are disposed on the opposite side of the DBR sensorlayers 2710 from the light emitting layers 2720. The photodetectorlayers 2730 are is configured to sense light emitted by the lightemitting layers 2720 and transmitted through the DBR layers 2710.

Assume that the reflective layers of the DBR sensor system 2700—compriseGaN and that initially the interstitial gaps contain air. Thereflectivity of the DBR layers 2710 would be relatively high due to thelarge refractive index difference between the GaN reflective layers andair. When the sensor system 2700 is placed in a test environment, ananalyte enters through the openings 2711 and replaces at least some ofthe air. The analyte has a different (higher) index of refraction thanair. Thus, the index of refraction difference between GaN and theanalyte contained in the interstitial gaps is lower than the GaN/airrefractive index difference. More of the light emitted by the lightemitting layers 2720 is transmitted through the DBR layers and reachesthe photodetector layers 2730. This alters the output signal 2631generated by the photodetector layers, indicating the presence of theanalyte.

Systems, devices or methods disclosed herein may include one or more ofthe features, structures, methods, or combinations thereof describedherein. For example, a device or method may be implemented to includeone or more of the features and/or processes described herein. It isintended that such device or method need not include all of the featuresand/or processes described herein, but may be implemented to includeselected features and/or processes that provide useful structures and/orfunctionality.

In the foregoing detailed description, numeric values and ranges areprovided for various aspects of the implementations described. Thesevalues and ranges are to be treated as examples only, and are notintended to limit the scope of the claims. For example, embodimentsdescribed in this disclosure can be practiced throughout the disclosednumerical ranges. In addition, a number of materials are identified assuitable for various facets of the implementations. These materials areto be treated as exemplary, and are not intended to limit the scope ofthe claims. The foregoing description of various embodiments has beenpresented for the purposes of illustration and description and notlimitation. The embodiments disclosed are not intended to be exhaustiveor to limit the possible implementations to the embodiments disclosed.Many modifications and variations are possible in light of the aboveteaching.

The invention claimed is:
 1. A sensor comprising: a plurality of layerpairs, each layer pair comprising: a reflective layer; and aninterstitial gap, each reflective layer having a first thickness and afirst index of refraction and each interstitial gap having a secondthickness and a second index of refraction, the first and secondthicknesses and the first and second indices of refraction selected toreflect incident light traveling along a first direction, the layerpairs arranged such that each interstitial gap is disposed between tworeflective layers, the plurality of layer pairs forming a distributedBragg reflector (DBR); and at least one opening configured to allowentry of an analyte into the interstitial gaps within the DBR.
 2. Thesensor of claim 1, wherein the sensor is a monolithic structure thatincludes a light emitting device configured to generate light that isdirected toward by the DBR.
 3. The sensor of claim 2, further comprisingan additional DBR, the light emitting device disposed between the DBRand the additional DBR.
 4. The sensor of claim 1, wherein the sensor isa monolithic structure that includes a photodetector configured todetect light reflected or transmitted by the DBR.
 5. The sensor of claim1, wherein the sensor is a monolithic structure that includes anadditional DBR and a light emitting device disposed between the DBR andthe additional DBR.
 6. The sensor of claim 5, wherein the light emittingdevice comprises a vertical cavity surface emitting laser.
 7. The sensorof claim 1, wherein air or a vacuum is present within the interstitialgaps before entry of the analyte.
 8. The sensor of claim 1, wherein thereflective layers comprise GaN.
 9. The sensor of claim 1, wherein thereflective layers are epitaxial layers.
 10. A system comprising: asensor, comprising: a plurality of layer pairs, each layer paircomprising: a reflective layer; and an interstitial gap, each reflectivelayer having a first thickness and a first index of refraction and eachinterstitial gap having a second thickness and a second index ofrefraction, the first and second thicknesses and the first and secondindices of refraction selected to reflect incident light traveling alonga first direction, the layer pairs arranged such that each interstitialgap is disposed between two reflective layers, the layer pairs forming adistributed Bragg reflector (DBR); and at least one opening configuredto allow entry of an analyte into the interstitial gaps; a lightemitting device configured to generate light directed toward the DBR;and a photodetector configured to sense light reflected or transmittedby the DBR and to generate a signal based on the sensed light.
 11. Thesystem of claim 10, further comprising a processor configured to receivethe signal and to detect a presence of the analyte based on the signal.12. The system of claim 11, wherein the processor is configured toidentify the analyte based on the signal.
 13. The system of claim 11,wherein the processor is configured to determine a characteristic of theanalyte based on the signal.
 14. A method comprising: exposing a sensorto an analyte, the sensor including: a plurality of layer pairs, eachlayer pair comprising: an interstitial gap, each reflective layer havinga first thickness and a first index of refraction and each interstitialgap having a second thickness and a second index of refraction, thefirst and second thicknesses and the first and second indices ofrefraction selected to reflect incident light traveling along a firstdirection, the layer pairs arranged such that each interstitial gap isdisposed between two reflective layers, the layer pairs forming adistributed Bragg reflector (DBR); and at least one opening fluidicallycoupled to the interstitial gaps; allowing the analyte to enter theinterstitial gaps through the opening, entry of the analyte changingoptical properties of the DBR; sensing light transmitted or reflected bythe DBR; and detecting a presence of the analyte based on the sensedlight.
 15. The method of claim 14, further comprising identifying theanalyte based on the sensed light.
 16. The method of claim 15, furthercomprising determining at least one characteristic of the analyte basedon the sensed light.
 17. The method of claim 16, wherein the at leastone characteristic comprises one or more of temperature, pressure andconcentration of the analyte.
 18. The method of claim 14, wherein thesensed light indicates a change in refractive index of the sensor whenthe analyte is present in the interstial gaps.
 19. The method of claim14, further comprising generating light directed toward the DBR.